Re: Where is Graham's mixer design? (was Re: RAT Tube Mixer update?)

John Byrns wrote:
> In article <fqf4uh$tam$1@[EMAIL PROTECTED]
>,
>  Ian Thompson-Bell <ruffrecords@[EMAIL PROTECTED]
> wrote:
> 
>> Eeyore wrote:
>>> Ian Thompson-Bell wrote:
>>>
>>>> Has anyone thought of using a grounded grid amplifier as a mix amp?
>>> No point as far as I can see.
>>>
>>> Graham
>>>
>> I can think of several.
>>
>> 1. Only needs one triode
>> 2. Input/output transfer characteristic is very linear when used with 
>> current source inputs
>> 3. No Miller effect.
>> 4. Input impedance approx 1/gm
> 
> Are you sure about that?  By my calculation that approximation only 
> holds as long as Ra is small compared with the load resistor.  Perhaps I

> made a mistake in my hand done calculations and should try to look the 
> result up in the RDH4th?
> 

I am not certain. I confess I copied the advantages mostly verbatim from 
J. Linsley Hood's book 'Valve and Transistor Amplifiers' which I have 
just managed to get from my local library. Approx 1/gm are his words and 
I have not looked at the underlying math.


>> 5. Gain approx anode load resistor over input resistor
> 
> Doesn't that gain approximation assume that Ra is low compared with the 
> value of the load resistor?
> 

Pass. Again it's a quote from JLH. I am pretty sure this book is a cut 
and paste jobs from other works of his because there are several 
formulae that include component references that do not appear in any of 
the accompanying diagrams.

> There seems to be a serious compromise here with respect to achieving a 
> low input resistance while maintaining the gain close to unity!
> 
>> Disadvantages are:
>>
>> 1.Heater cathode leakage can inject hum/noise into input path.
> 
> Is this a serious problem assuming a DC heater supply is used, and the 
> heater bus is well filtered/isolated to prevent the entry of other noise

> sources?
> 
> 


Using well filtered DC supplies I agree it should be a concern.

I am not sure it is a particularly good idea, it was just that as I read 
the section in the JLH book it struck me it had many of the requirements 
of a mix amp, hence my question had anyone considered it as such.

Cheers

Ian